1. Field of the Invention
This invention relates generally to the field of trunked radio systems and specifically to a method and means for automatically maintaining critical system parameters in a trunked radio system which provides for automatic interconnect to a conventional public switched telephone system.
2. Description of Prior Art
Trunked radio systems have developed as a means of providing increased user mobile unit capacity in systems which operate in a fixed geographic area. In the past a typical radio system was comprised of a base station transmitting and receiving on a single frequency and a fleet of mobile units operating on that same frequency. The capacity of this type of radio system is severely limited by the number of users and the average length of calls on those frequencies. Trunked systems alleviate the constraints of the single frequency systems by operating on several alternative frequencies, and communication between a base unit and mobile is done on channels which are dynamically assigned by a control unit on a separate control channel. In effect, trunking systems provide for automatic channel assignment in a land mobile radio. Trunked ratio systems improve both spectral efficiency and operating characteristics of land mobile radio systems.
Trunked radio systems are operated with stringent system constraints. The most important of these constraints is the amount of time in which a dispatch call request must wait to be assigned to an operative radio frequency. The dispatch access time period is affected by the number and duration of all types of calls which are operative on the system at any particular time. The dispatch access time parameter is especially critical in light of recent developments toward interconnecting conventional telephone systems to trunked radio systems (FCC docket 20846). The characteristics of telephone interconnect traffic are very different from those of trunked radio dispatch traffic. Research done by the Federal Communications Commission has shown that the average message length for interconnect calls is approximately 119 seconds and our research has shown that the average message length for dispatch telephone calls is approximately 21.9 seconds. The effects of long message lengths for telephone traffic are manifested in the "system access delay" that a user experiences when placing a call on a busy system. System access delay may be thought of as the time that a user waits for a free channel, as a function of system loading.
A pure dispatch trunked system can accommodate a larger number of users per channel at a given access delay than can a pure mobile telephone system. Stated alternatively, a user of a mobile telephone system having a certain number of users per channel will experience a relatively longer access delay than the user of a pure dispatch system with the same number of users per channel. If telephone traffic is indiscriminately mixed with dispatch traffic on a trunked radio system, the possibility exists that dispatch system access delay will be degraded to that of the pure mobile telephone system.